What are optical illusions

Im sure every single one has seen one of this pictures that really mess up with your eyes and  your brain.
The so called visual or optical illusions are characterized by visually perceived images that differ from objective reality.



There are three main types:

1.  Literal optical illusions that create images that are different from the objects that make them
2.  Pshyiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, colour, size, position, tilt, movement)
3. Cognitive illusions, the result of unconscious inferences.

Here are some off optical illussions below :